doc: Enumerate threats in model

Certain external requirements require justification that threats in a
threat model have been satisfied.  In order to do this, the threats must
be enumerated, and given labels.

Add labels to the threats.  Use rts's citation model to allow the
threats to be grouped and listed at the end of the document to help with
cross referencing.

Signed-off-by: David Brown <david.brown@linaro.org>